Fiber optic cables are currently being used throughout the aerospace and communications industries. These cables are typically employed in computerized equipment and communications applications where space and/or weight restrictions make traditional copper wiring systems inappropriate. Optical fiber, as a data transfer means, is known for its exceptional speed and bandwidth capability and for its ability to provide reliable communication signals.
The commercial and military aerospace industry is a harsh testing ground for fiber optic cables, requiring flawless performance in extremely demanding physical environments. In such environments, where even minor failures can result in loss of life and property, fiber optic cables are subjected to conditions which include destructive extremes in vibration, shock, temperature, pressure, water/chemical emersion, as well as, electromagnetic and radio frequency interference. It is for these reasons that aerospace standards for the manufacture and supply of fiber optic cables are some of the most rigorous found in any industry.
The Boeing Company (“Boeing”), as an aircraft manufacturer serving a worldwide market, maintains fiber optic cable standards for, among other things, smoke and toxic gas emissions, cable jacket shrinkage and finished cable attenuation.
Test methods detailed in Section 7.46 of Boeing Standard BSS-7324 entitled “Procedure for Testing Electrical Wire and Cable”, dated Aug. 18, 1998 (“Boeing BSS-7324”), which are incorporated herein by reference, relate to smoke emission and toxicity. Pursuant to these tests, a fiber optic cable sample (3.05 meter sample) is burned for four minutes in a burn chamber under both flaming and non-flaming conditions. To meet the smoke emission standard, the specific optical density (Ds) of the resulting smoke must be less than 50. To meet the toxicity requirements for smoke gases, the following six gases must not be present in the smoke contained in the chamber in quantities at or above the quantities listed below:
parts per million (ppm)Carbon Monoxide (CO)3500Hydrogen Cyanide (HCN)150Hydrogen Fluoride (HF)200Hydrogen Chloride (HCl)500Sulfur Dioxide (SO2 + H2S)100Nitrous Gases (NO + NO2)100
Test methods detailed in Section 8.1.1 of Boeing Standard BMS-71 entitled “Draft BMS 13-71 Cable, Fiber Optic”, dated Mar. 23, 2002 (“Boeing BMS 13-71”), in Judd Wire, Inc.'s Standard Operating Procedure (SOP) Number 90111, entitled “Shrinkage Measurement Method”, publication date—Oct. 29, 2003 (“Judd SOP 90111”), and in Electronic Industries Association (EIA)/Telecommunications Industry Association (TIA) Test Procedure Number 455-3A, dated May 23, 1989 (“EIA/TIA Test Procedure Number 455-3A”), which are all incorporated herein by reference, relate to cable jacket shrinkage and optical attenuation stability in the finished cable. Pursuant to the above-referenced tests, a fiber optic cable is exposed to a temperature cycling regimen using a dynamic mechanical analyzer (DMA) with zero load and the degree of jacket shrinkage, as well as, the stability of optical attenuation in the cable measured. To meet the standards, the degree of cable jacket shrinkage must not exceed a 45 millimeter (mm) (1.3%) maximum change, while the finished cable attenuation for fiber optic cables employing one or more 62.5/125 μm graded-index, multi-mode optical fibers must not exceed 3.5 decibels per kilometer (dB/km) at 850 nanometers (nm), and 2.0 dB/km at 1300 nm.
In addition to recognized aerospace standards for the manufacture and supply of fiber optic cables, it is noted that the aerospace industry, in its quest for new designs and materials that can deliver stronger, lighter and more durable fiber optic cables, has recently placed a strong emphasis on small form factor optical connectors such as LC connectors, which are available from Lucent Technologies, Inc. The LC connector employs a ceramic ferrule having a diameter that is only 1.25 millimeters. The use of LC connectors requires bonding of the fiber optic cable to the inside of the ferrule. Unfortunately, of the limited number of materials likely to meet the smoke and toxic gas generation standards detailed above (i.e., fluoropolymers and polyimides), fluoropolymers, especially as they approach the perfluorinated state, are extremely difficult to effectively bond to any surface.
Prior art attempts to satisfy the rigorous aerospace fiber optic cable standards, as well as, address the optical connector interfacing challenges, which are noted above, include a ruggedized fiber optic cable described in U.S. Pat. No. 6,233,384 B1 to Sowell, III et al. The ruggedized fiber optic cable is prepared by applying a fluoropolymer first jacketing material over a buffered optic fiber core. A rigid, closely-spaced, spirally or helically wrapped wire layer is then applied over the fluoropolymer first jacketing layer, followed by the application of a mechanical braid (e.g., plastic fibers or strands) over the wire layer. To protect the fiber optic cable from the environment, an outer jacket (e.g., a tetrafluoroethylene/(perfluoroalkyl) vinyl ether copolymer) is applied over the mechanical braid. Although this cable design provides some protection for the optic fiber core, the outer jacket will shrink in the axial direction during cable manufacture and use, thereby increasing stress on the optic fiber core, which can cause the fibers to crack or break. In addition, bonding this cable to LC connectors would be difficult where the fluoropolymers used to form the first jacketing material include perfluorinated polymers (e.g., PTFE) which, as noted above, are extremely difficult to effectively bond to any surface. Further, it is noted that the use of steel and other metallic wires in this cable design results in a substantial increase in weight, which is objectionable in aerospace applications.
U.S. Pat. No. 5,615,293 to Sayegh discloses a fiber optic cable assembly that employs acrylic coated optical fibers surrounded by a buffer material such as foamed fluorinated ethylene-propylene (FEP). The acrylic coating material on the optical fibers, however, has a use temperature ranging from about −65° C. to about 125° C. and will degrade when the FEP buffer material, which must be melt-processed at a temperature exceeding 300° C., is extruded onto the fibers, thereby causing undesirable yellowing and even loss of integrity of the coating material. In addition, of the embodiments described in this reference, many would fail to satisfy the rigid smoke and toxic gas emission standards noted herein while others would not bond effectively to optical connectors.
Accordingly, it is a general object of the present invention to avoid the above-referenced disadvantages of the prior art.
More particularly, it is an object of the present invention to provide a fiber optic cable that provides an intermediate surface that facilitates bonding to optical connectors such as LC connectors.
It is yet a more particular object to provide a low smoke, low toxicity fiber optic cable that facilitates bonding to optical connectors and that exhibits improved dimensional stability by minimizing or eliminating shrinkage stress on the optic fiber core, thereby demonstrating more stable signal carrying characteristics in extremely demanding physical environments.